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Article

Enhanced Photocatalytic Hydrogen Production of ZnIn2S4 by Using Surface-Engineered Ti3C2Tx MXene as a Cocatalyst

by
Mengdie Cai
*,†,
Xiaoqing Zha
†,
Zhenzhen Zhuo
,
Jiaqi Bai
,
Qin Wang
,
Qin Cheng
,
Yuxue Wei
and
Song Sun
*
School of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Materials 2023, 16(6), 2168; https://doi.org/10.3390/ma16062168
Submission received: 16 February 2023 / Revised: 3 March 2023 / Accepted: 6 March 2023 / Published: 8 March 2023
(This article belongs to the Special Issue Recent Advances in Nanocomposite Materials for Photocatalytic and Electrocatalytic Hydrogen Production)
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Abstract

:
Developing efficient and stable photocatalysts is crucial for photocatalytic hydrogen production. Cocatalyst loading is one of the effective strategies for improving photocatalytic efficiency. Here, Ti3C2Tx (Tx = F, OH, O) nanosheets have been adopted as promising cocatalysts for photocatalytic hydrogen production due to their metallic conductivity and unique 2D characterization. In particular, surface functionalized Ti3C2(OH)x and Ti3C2Ox cocatalysts were synthesized through the alkalization treatment with NaOH and a mild oxidation treatment of Ti3C2Fx, respectively. ZnIn2S4/Ti3C2Tx composites, which were fabricated by the in-situ growth of ZnIn2S4 nanosheets on the Ti3C2Tx surface, exhibited the promoted photocatalytic performance, compared with the parent ZnIn2S4. The enhanced photocatalytic performance can be further optimized through the surface functionalization of Ti3C2Fx. As a result, the optimized ZnIn2S4/Ti3C2Ox composite with oxygen functionalized Ti3C2Ox cocatalyst demonstrated excellent photocatalytic hydrogen evolution activity. The characterizations and density functional theory calculation suggested that O-terminated Ti3C2Ox could effectively facilitate the transfer and separation of photogenerated electrons and holes due to the formation of a Schottky junction, with the largest difference in work function between ZnIn2S4 and Ti3C2Ox. This work paves the way for photocatalytic applications of MXene-based photocatalysts by tuning their surface termination groups.

1. Introduction

Hydrogen is regarded as an ideal energy with the advantages of a high energy capacity and zero pollutants. Among the various H2 production strategies, solar-light-driven photocatalysis for H2 production from water splitting is a promising route to alleviating the energy crisis [1,2,3]. Developing highly efficient photocatalysts is the key to realizing the industrialization of photocatalytic H2 production. Regarding photocatalysts, ZnIn2S4 has attracted more attention in recent years because of its low toxicity, visible-light response, and considerable photostability [4,5]. However, the rapid recombination and tardy migration of the photogenerated electrons and holes restricts the photocatalytic H2 production efficiency of bare ZnIn2S4 [6,7]. To address this issue, diverse approaches, including cocatalyst loading, vacancy engineering and heterojunction construction, have been systematically developed to improve the photocatalytic performance of ZnIn2S4 materials [8,9,10]. Among them, cocatalyst loading has been verified to be a feasible and efficient method to promote the photocatalytic efficiency by accelerating the separation and transfer of photogenerated charge carriers while simultaneously acting as active sites to facilitate the photocatalytic H2 production reaction kinetics. The employment of noble metals (such as Pt, Au, Pd and Rh) as cocatalysts, has been proven to be highly efficient in improving the photocatalytic performance, but their high price largely limits their widespread application [11,12]. Therefore, it is urgent to explore an inexpensive and efficient noble metal-free cocatalyst to replace Pt, Au, Pd and Rh to achieve large-scale photocatalytic H2 production.
MXene, as an emerging family of 2D transition metal carbides/nitrides, has gained intensive scientific interest in photocatalysis, ascribed to its excellent metal conductivity, large specific surface area with abundant active sites and hydrophilicity [13,14,15]. The 2D planar structure of Ti3C2Tx MXene is beneficial to highly dispersing the host photocatalyst with a strong interfacial contact [16,17,18]. On the other hand, owing to its high conductivity and abundant exposed metal sites, Ti3C2Tx could act as a cocatalyst to facilitate the separation and migration of photogenerated charge carriers and lower the reaction energy barriers for accelerating the reaction kinetics. Therefore, Ti3C2Tx was widely used as a cocatalyst in photocatalytic H2 production [18,19,20]. For instance, Zhao et al. [17] reported the construction of hierarchical 2D Bi2MoO6@Ti3C2Tx by in-situ growing Bi2MoO6 onto the surface of Ti3C2Tx nanosheets. Ti3C2Tx, as the cocatalyst, could not only suppress the agglomeration of Bi2MoO6 nanosheets and increase the reaction active sites, but also endow the photocatalyst with the Schottky junction. As a result, the Bi2MoO6@Ti3C2Tx exhibited enhanced photocatalytic activity. Zuo et al. [18] found that the ZnIn2S4-Ti3C2Tx-ZnIn2S4 sandwich-like hierarchical heterostructures exhibited a superior photocatalytic H2 production performance due to the construction of the Schottky junction between ZnIn2S4 nanosheets and Ti3C2Tx. Ran et al. [19] reported that Ti3C2, as a potential cocatalyst, could efficiently improve the photocatalytic hydrogen production performance by forming the Schottky junction at the Ti3C2/CdS interface to facilitate the separation of the photogenerated electrons and holes. Meanwhile, they found that the Gibbs free energy for H adsorption (ΔGH*) of O-terminated Ti3C2 is close to zero. With the near-zero ΔGH*, the favorable Fermi level position and electrical conductivity, O-terminated Ti3C2 could serve as an alternative to noble metals in photocatalytic H2 production. Liu et al. [20] utilized Ti3C2 nanosheets acting as the substrate and cocatalyst to synthesize a CdLa2S4/Ti3C2 photocatalyst, which could not only promote the dispersion of CdLa2S4, but also enhance the photogenerated charge carriers separation and transfer, leading to a significant enhancement in photocatalytic H2 evolution. In most cases, Ti3C2 with a large work function could act as electron sink to facilitate the separation and transfer of the photogenerated charge carriers in photocatalytic H2 production. In contrast, Peng et al. [21] proposed a dual-carrier-separation mechanism for photocatalytic H2 evolution within Cu/TiO2@Ti3C2Tx, where -OH-terminated Ti3C2Tx with a lower work function than TiO2 served as the hole trap to accelerate the holes migration from TiO2 to Ti3C2Tx. Obviously, the surface termination groups of Ti3C2Tx could arise tunable electronic properties (such as work function) to impact on the photocatalytic performance of the Ti3C2Tx-based photocatalysts.
Tailoring the surface termination groups of Ti3C2Tx could alter their work function, electronic and optoelectronic properties [22,23,24]. Recently, the theoretical calculations from Khazaei revealed that the work function of Ti3C2Tx was strongly dependent on the surface termination groups, and the work function of Ti3C2Tx could adjust in a wide range from 1.6 eV to 6.0 eV [24]. Jiang et al. [25] investigated the effect of the surface terminations of Ti3C2Tx on the electrocatalytic H2 evolution. They found that O-terminated Ti3C2Tx nanosheets exhibited much higher H2 evolution activity than other Ti3C2Tx, and the –O termination groups on the basal plane of Ti3C2 were the H2 evolution reaction active sites. Especially, the –O termination groups could promote the adsorption of H and accelerate the H2 evolution reaction. However, the insights into the effect of the surface termination groups in Ti3C2Tx MXene-based photocatalysts on the photocatalytic H2 production are not established experimentally. Herein, we designed a series of Ti3C2Tx (Tx = F, OH, O) with different surface termination groups, and then the 2D ZnIn2S4 was in-situ grown on the surface of Ti3C2Tx using a facile hydrothermal synthesis method to synthesize ZnIn2S4/Ti3C2Tx composites. Specifically, the as-synthesized ZnIn2S4/Ti3C2Ox with the O-terminated Ti3C2Tx exhibited the superior photocatalytic H2 production activity. When the content of Ti3C2Ox was 1.0 wt%, the ZnIn2S4/Ti3C2Ox presented the optimal photocatalytic H2 production rate of 363 μmol g−1 h−1. This work provides us with a paradigm for the rational design of Ti3C2Tx MXene with tailored surface termination groups and the development of efficient MXene-based composites for photocatalytic applications.

2. Materials and Methods

2.1. Samples Preparation

2.1.1. Synthesis of Ti3C2Fx

Typically, 2 g LiF was added into 40 mL HCl aqueous solution (9 M) and stirred for 1 h until the LiF was completely dissolved. A total of 2 g of Ti3AlC2 powder was then added to the above solution and stirred for 0.5 h. The suspension was stirred at 53 °C for 41 h. Upon cooling, the mixture was centrifuged and washed with deionized water until the pH was close to 7. The product was dried at 60 °C under vacuum for 48 h.

2.1.2. Synthesis of Surface Functionalized Ti3C2Tx

In order to obtain the surface functionalized Ti3C2Tx, the pristine Ti3C2Fx were treated with a different functionality-modification strategy. To achieve Ti3C2(OH)x with −OH rich termination groups, according to the previous literature [25], 0.2 g of the pristine Ti3C2Fx was dispersed in 100 mL of 1 M NaOH aqueous solution in order to replace the −F surface termination groups with −OH. After stirring for 2 h at room temperature, the product was centrifuged and washed with deionized water until the pH was close to 7. Then, the product was collected and dried at 60 °C under vacuum for 12 h. To obtain O-terminated Ti3C2Ox, the Ti3C2Fx was calcined under 300 °C in Ar gas flow for 2 h.

2.1.3. Synthesis of ZnIn2S4 and ZnIn2S4/Ti3C2Tx

Typically, 0.176 g InCl3·4H2O, 0.041 g ZnCl2 and 0.120 g thioacetamide (TAA) were added consecutively into 40 mL glycerol aqueous solution (20 vol%) and stirred for 0.5 h. The quantitative Ti3C2Tx (Tx = F, OH, O) (1.0 wt%) was added into the above solution. The mixed suspension was heated at 80 °C with stirring for 2 h. After centrifugation, the products were collected and dried at 60 °C for 12 h.
For comparison, the preparation of the pristine ZnIn2S4 was similar to that of ZnIn2S4/Ti3C2Tx without the introduction of Ti3C2Tx.

2.2. Photocatalytic H2 Production Experiments

The photocatalytic H2 production tests were carried out in a Pyrex glass reaction (Beijing Perfectlight Labsolar-6A, Perfectlight Technology, Beijing, China) with a circulated cooling water system to maintain the temperature at 8 °C. A total of 100 mg of the photocatalyst was dispersed in 100 mL aqueous solution, containing 10 vol% triethanolamine (TEOA) as the sacrificial agent. Before irradiation under a Xe lamp (CEL-HXUV300, Perfectlight Technology, Beijing, China), the suspension was evacuated by the vacuum pump. The produced H2 volume was analyzed using an on-line gas chromatograph (GC5190, TCD, A column, Ar carrier).

2.3. Characterization

The powder X-ray diffraction (XRD) patterns of the prepared samples were collected using a Rigaku SmartLab (9 kW, Tokyo, Japan) diffractometer with Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 4 mA. The morphological analysis of the samples were recorded with scanning electron microscopy (SEM) using a Regulus 8230 scanning electron microscope (Hitachi, Ltd., Tokyo, Japan) at an acceleration voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) was carried out to investigate the surface chemical environment of the samples using an Escalab 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Measuring with an ultraviolet photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) was performed with a −5 V bias voltage. The data were calibrated with a C1s spectrum of 284.6 eV. The Fourier transform spectrophotometer (Vertex80 + Hyperion2000, Bruker, Billerica, MA, USA) was employed to acquire IR spectra with the standard KBr disk method. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of the samples were collected with a field-emission electron microscope (JEM-2100F, JEOL, Tokyo, Japan). UV-visible diffuse reflectance spectroscopy (UV-vis DRS) was recorded to study the optical absorption ability of photocatalysts with Hitachi U-4100 UV-visible spectrometer using a reference standard of BaSO4. The photoluminescence (PL) spectra and time-resolved fluorescence spectra were conducted on an Edinburgh FLS 1000 spectrometer (Edinburgh Instruments Ltd., Livingstone, UK) over an exaction wavelength of 375 nm. Electrochemistry impedance spectroscopy (EIS), Mott–Schottky analyses and transient photocurrent spectra were measured using a CHI660E analyzer (CH Instruments, Inc., Bee Cave, TX, USA) with a standard three-electrode system.

3. Results

3.1. Schematic Illustration of the Synthesis

The schematic illustration in Scheme 1 shows the synthesis process for the ZnIn2S4/Ti3C2Tx (Tx = F, OH, O) samples, which consists of three steps: the preparation of the Ti3C2 by the selective etching of Ti3AlC2, surface post-treatment (the alkalization treatment with NaOH and the mild oxidation treatment with Ar calcination) of Ti3C2Fx to replace the –F termination groups with –OH or –O groups and the in-situ hydrothermal synthesis of ZnIn2S4 on surface of Ti3C2Tx.

3.2. Samples Characterization

The X-ray diffraction (XRD) patterns of Ti3AlC2 and the as-prepared Ti3C2Tx (Tx = F, OH, O) samples in Figure S1 demonstrated a typical Ti3AlC2 and Ti3C2Tx MXene phase. No crystal structure variation was observed for the Ti3C2(OH)x and Ti3C2Ox, indicating that the surface functionalization treatments just modulated the termination groups without changing the crystalline structure of Ti3C2Fx. The XRD pattern of Ti3C2Ox showed no peaks of TiO2. Meanwhile, the morphology of the Ti3C2Tx nanosheets was maintained even after the alkalization and oxidation treatments (Figure S2).
To confirm the surface termination groups of the as-prepared Ti3C2Tx (Tx = F, OH, O) samples, we performed X-ray photoelectron spectroscopy (XPS), as shown in Figure 1a–c. Figure 1a showed the high-resolution XPS spectrum of F 1s, the binding energy at 685.8 eV was assigned to the Ti-F bond [26]. After the alkalization treatment and mild oxidation treatment of Ti3C2Fx, the Ti-F peak intensity in Ti3C2(OH)x and Ti3C2Ox both significantly decreased, indicating that the surface functionalization treatments did not change its crystal structure, while the termination groups had modulated noticeably. The elemental composition result determined by XPS (Table S1) also confirmed the decrease of the –F termination groups. As seen from the Ti 2p XPS spectra in Figure 1b, more detailed structural variation could be obtained, four doublets were fitted for Ti 2p3/2 and Ti 2p1/2, which indicated that the Ti species in Ti3C2Tx exhibited four kinds of chemical environment. The Ti 2p3/2 binding energies at approximately 455.1, 455.8, 456.9 and 459.1 eV could be assigned to C-Ti-C, C-Ti-OH, C-Ti-O and O-Ti-O bonds, respectively [23,27,28]. Obviously, compared to Ti3C2Fx, the intensity of the C-Ti-O peak for Ti3C2Ox increased and the intensity of the C-Ti-OH peak for Ti3C2(OH)x increased, which indicated that the –F terminations in the Ti3C2Fx were replaced by –O and –OH after the oxidation treatment and alkalization treatment, respectively. The intensity of the O-Ti-O peak increased in Ti3C2Ox and Ti3C2(OH)x, which was attributed to the surface oxidation with the transform C-Ti-C band to O-Ti-O. Furthermore, the O 1s XPS spectra (Figure 1c) exhibited Ti-O, Ti-OH and C-OH peaks at the binding energies of 530.1, 531.8 and 533.5 eV, respectively [29,30]. In particular, the peak at 531.8 eV demonstrated the highest proportion of –OH groups on the surface of Ti3C2(OH)x, while Ti3C2Ox showed the highest concentration of Ti-O due to O-terminated surfaces. The XPS results showed the coexistence of Ti-F, Ti-OH and Ti-O bonds in all Ti3C2Tx samples. It should be noted that the Ti3C2Ox, Ti3C2(OH)x and Ti3C2Ox represented a higher density of termination groups –F, –OH and –O on the surface, respectively. After the alkalization treatment, the Ti-F peak intensity significantly decreased while the Ti-OH peak intensity increased in Ti3C2(OH)x, implying the successful replacement of –F with –OH. Similarly, the –F groups in Ti3C2Fx were successfully replaced by –O with the mild oxidation treatment to form the Ti3C2Ox.
The surface termination groups of the Ti3C2Tx samples were further analyzed using Fourier transform infrared spectroscopy (FTIR), as displayed in Figure 2d. The FTIR spectrum of Ti3C2Tx samples showed two peaks at approximately 3430 and 1625 cm−1, which assigned to the −OH band on the surface of Ti3C2Tx. In addition, a peak at 657 cm−1 can be observed, which is attributed to the Ti-O band [31]. It is notable that Ti3C2(OH)x showed the strongest −OH vibration intensity and that the Ti3C2Ox exhibited a significantly increased Ti-O vibration, which was consistent with the XPS results. These results indicated that the surface functionalized Ti3C2(OH)x and Ti3C2Ox were successfully synthesized with the alkalization treatment and mild oxidation treatment, respectively.
The ZnIn2S4/Ti3C2Tx (Tx = F, OH, O) composites were obtained through the in-situ growth of ZnIn2S4 onto the surface of Ti3C2Tx. To acquire the crystallinity phase of the ZnIn2S4 and ZnIn2S4/Ti3C2Tx composites, the XRD analysis was introduced (Figure S3). It was found that all ZnIn2S4/Ti3C2Tx samples presented similar diffraction peaks with ZnIn2S4. The missing Ti3C2Tx diffraction peaks could be ascribed to the low content and high dispersion of Ti3C2Tx in the composites. The morphology of the ZnIn2S4 and ZnIn2S4/Ti3C2Ox samples were investigated using scanning electron microscopy (SEM). The ZnIn2S4 presented a morphology of nanoflowers stacked with nanosheets (Figure S4). From the SEM image of ZnIn2S4/Ti3C2Ox sample in Figure 3a, it can be seen that the ZnIn2S4 particles are uniformly dispersed and anchored onto the Ti3C2Ox surface. The more detailed microstructure of the ZnIn2S4/Ti3C2Ox composite were further demonstrated using the transmission electron microscopy (TEM) technique. TEM observation confirmed such hierarchical ZnIn2S4/Ti3C2Ox structure (Figure 2b). Furthermore, as shown in Figure 2c, the lattice distances of the ZnIn2S4/Ti3C2Ox photocatalyst were measured, and the lattice fringes spacing of 0.32 and 0.41 nm were corresponded to the (102) and (006) planes of ZnIn2S4, while the lattice fringes spacing of 0.26 nm was assigned to the (0110) crystal plane of Ti3C2Ox. Moreover, there was an obvious interface contact observed between the ZnIn2S4 and the Ti3C2Ox, which could contribute to the faster transfer of the photogenerated charge. In addition, the corresponding EDX elemental mapping (Figure 2d) displayed that the Zn, In, S, Ti and C elements were uniformly distributed in the ZnIn2S4/Ti3C2Ox sample. The above results powerfully confirmed that the ZnIn2S4/Ti3C2Ox photocatalyst was successful constructed.
The optical properties of pristine ZnIn2S4 and ZnIn2S4/Ti3C2Fx (T = F, OH, O) composites were analyzed using the UV-vis diffuse reflectance spectra (UV-vis DRS). As shown in Figure 3a, the pristine ZnIn2S4 showed an absorption edge at 560 nm, while the absorption edge of the ZnIn2S4/Ti3C2Tx composites exhibited a slightly red shift with the introduction of Ti3C2Tx. Moreover, compared to that of ZnIn2S4, the absorption intensities of the ZnIn2S4/Ti3C2Tx composites increased in the whole visible light region, suggesting that the Ti3C2Tx loading increased the visible light utilization efficiency of ZnIn2S4. In addition, the UV-vis DRS spectra of ZnIn2S4 and ZnIn2S4/Ti3C2Tx composites were converted into Tauc’s band gap plots (Figure 3b), the band gaps of ZnIn2S4, ZnIn2S4/Ti3C2Fx, ZnIn2S4/Ti3C2(OH)x and ZnIn2S4/Ti3C2Ox were measured to be 2.64 eV, 2.60 eV, 2.59 eV and 2.63 eV, respectively.

3.3. Photocatalytic H2 Evolution Activity

The photocatalytic H2 evolution activity of the as-obtained pure ZnIn2S4 and ZnIn2S4/Ti3C2Tx composites were evaluated under visible light irradiation using triethanolamine (TEOA) as a sacrificial reagent. It was well known that Ti3C2Tx were not semiconductors and they could not generate electrons and holes upon light irradiation [32]. Therefore, Ti3C2Tx had no photocatalytic H2 evolution activity. In Figure 4a, the pure ZnIn2S4 exhibited the poor H2 evolution rate of 253 μmol h−1 g−1. Inspiringly, after loading the Ti3C2Tx cocatalysts, the ZnIn2S4/Ti3C2Tx composites all exhibited the improved photocatalytic H2 evolution activity, and the order of photocatalytic activity was ZnIn2S4/Ti3C2Ox > ZnIn2S4/Ti3C2Fx > ZnIn2S4/Ti3C2(OH)x > ZnIn2S4. Furthermore, the photocatalytic H2 evolution rate of the ZnIn2S4/Ti3C2Ox composites strongly depended on the amount of Ti3C2Ox. The ZnIn2S4/Ti3C2Ox composite with 1.0 wt% Ti3C2Ox achieved the optimal H2 evolution rate of 363 μmol h−1 g−1 (Figure 4b). By further increasing the Ti3C2Ox content, the H2 evolution rate of the ZnIn2S4/Ti3C2Ox composite decreased, which could be due to the excessive amount of Ti3C2Ox covering the active sites and hindering the light absorption of ZnIn2S4 [33]. The photocatalytic stability test of ZnIn2S4/Ti3C2Ox for photocatalytic H2 evolution was carried out for four consecutive cycles (Figure 4c). It can be seen that ZnIn2S4/Ti3C2Ox maintained the photocatalytic H2 evolution activity during the four consecutive cycles, indicating the excellent photostability of ZnIn2S4/Ti3C2Ox.

3.4. The Mechanism of Enhanced Photocatalytic Activity

To shed light on the fundamental reasons for the enhanced photocatalytic performance of ZnIn2S4/Ti3C2Ox, fluorescence property and photoelectrochemical measurements were employed. It is well known that the transfer efficiency of photogenerated electrons and holes was an important influencing factor for the photocatalytic performance. The photoluminescence (PL) spectrum was employed to illustrate the transfer efficiency of the photogenerated electrons and holes. Figure 5a showed the PL spectra of the ZnIn2S4 and ZnIn2S4/Ti3C2Tx composites measured at 375 nm. The order of the PL signal intensity at 565 nm was ZnIn2S4 > ZnIn2S4/Ti3C2(OH)x > ZnIn2S4/Ti3C2Fx > ZnIn2S4/Ti3C2Ox. The loading of Ti3C2Tx lead to the decreased PL intensity of ZnIn2S4, and the ZnIn2S4/Ti3C2Ox composite showed the lowest PL intensity, which indicated that the addition of the Ti3C2Ox cocatalyst could effectively facilitate the transfer of the photogenerated electrons and hole on the ZnIn2S4 photocatalyst. The time-resolved photoluminescence (TRPL) spectra (Figure 5b) further certified this result. The calculated average fluorescence lifetime (Ave. τ) of ZnIn2S4/Ti3C2Ox (0.594 ns) was significantly longer than that of ZnIn2S4 (0.167 ns), which demonstrated that the Ti3C2Ox cocatalyst loading greatly reduced the recombination rate of the photogenerated electrons and holes on ZnIn2S4. In addition, electrochemical impedance spectroscopy (EIS) and transient photocurrent response analyses were carried out to further investigate the separation and transfer ability of the photogenerated charge carriers. The EIS Nyquist plots were shown in Figure 5c and the arc radius on the EIS Nyquist plot of ZnIn2S4/Ti3C2Ox was the smallest among these four samples, which indicated its lowest resistance for the charge carriers on the ZnIn2S4/Ti3C2Ox composite. This also confirmed that the Ti3C2Ox cocatalyst enhanced the separation and transfer efficiency of the photogenerated electrons and holes of ZnIn2S. The transient photocurrent densities of the as-prepared samples were displayed in Figure 5d. Compared with that of the blank ZnIn2S4, the photocurrent densities of the ZnIn2S4/Ti3C2Tx samples exhibited remarkable increases; in particular, ZnIn2S4/Ti3C2Ox exhibited the highest photocurrent density, further confirming the excellent photogenerated carriers transfer and separation ability of ZnIn2S4/Ti3C2Ox. All of these results proved that the ZnIn2S4/Ti3C2Ox exhibited the fastest transfer and separation ability of photogenerated electrons and holes, further resulting in the excellent photocatalytic H2 production performance.
In terms of the band theory, electron transfer behavior is closely related to the work functions of ZnIn2S4 and Ti3C2Tx (Tx = F, OH, O). In order to determine the work functions (Φ) of the ZnIn2S4 andTi3C2Tx samples, the ultraviolet photoelectron spectroscopy (UPS) technique was employed, as shown in Figure 6. The incident photon energy (hν) was 21.22 eV. As for ZnIn2S4 (Figure 6a), the secondary electron cutoff energy (Ecutoff) was 9.32 eV and the Fermi energy (EFermi) was 25.92 eV. The work function of ZnIn2S4 was calculated to be 3.33 eV using the formula: Work function (WF) = hν + Ecutoff − EFermi. Similarly, the work functions for Ti3C2Fx, Ti3C2(OH)x and Ti3C2Ox were calculated to be 4.22 eV, 3.73 eV and 4.57 eV, respectively (Figure 6b–d). Obviously, the work functions of the Ti3C2Tx samples were all higher than that of ZnIn2S4. Therefore, the photogenerated electrons could transfer from ZnIn2S4 to Ti3C2Tx. Meanwhile, the Schottky barrier could be formed at the ZnIn2S4/Ti3C2Tx interface due to the difference in the work function and the band alignment between ZnIn2S4 and Ti3C2Tx, which could greatly accelerate the separation and transfer of the photogenerated electrons and holes [34]. The electrostatic potentials of Ti3C2Fx, Ti3C2(OH)x and Ti3C2Ox were obtained from a density functional theory (DFT), as shown in Figure S5. The order of work function values obtained from the DFT calculations was in accordance with that from the UPS characterization. Moreover, the difference in work function between ZnIn2S4 and Ti3C2Tx was associated with the photogenerated electrons’ transfer ability [35,36]. The largest difference in the work function between ZnIn2S4 and Ti3C2Ox indicated that Ti3C2Ox showed the strongest electron capture capability from ZnIn2S4 in the ZnIn2S4/Ti3C2Ox heterojunction, leading to the significantly high photocatalytic activity.
Based on the aforementioned results, a probable photocatalytic mechanism for ZnIn2S4/Ti3C2Ox was proposed (Figure 7). The conduction band potential of the parent ZnIn2S4 was estimated by the Mott-Schottky method (Figure S6). Under visible light irradiation, the photogenerated electrons on the valence band (VB) of ZnIn2S4 were excited to the conduction band (CB). Because the work function of Ti3C2Ox was higher than that of ZnIn2S4, photogenerated electrons in the CB of ZnIn2S4 could quickly migrate to the surface of Ti3C2Ox across the intimate interface, the Schottky junction formed between ZnIn2S4 and Ti3C2Ox could further prevent the recombination of photogenerated electrons and holes in the ZnIn2S4/Ti3C2Ox. Subsequently, the photogenerated electrons in ZnIn2S4/Ti3C2Ox were available to react with water to evaluate H2, while the holes on the VB of ZnIn2S4 are consumed by the sacrificial agent TEOA.

4. Conclusions

In summary, we have successfully designed and synthesized the surface functionalized Ti3C2(OH)x and Ti3C2Ox using the surface post-treatments of Ti3C2Fx; then Ti3C2Tx (Tx = F, OH, O) were employed as the substrate and cocatalysts for the in-situ growth of ZnIn2S4 to obtain ZnIn2S4/Ti3C2Tx heterojunctions for photocatalytic H2 production. Remarkably, the photocatalytic H2 production activity of ZnIn2S4/Ti3C2Tx was greatly improved, compared to that of ZnIn2S4. Due to the differences in work function between ZnIn2S4 and Ti3C2Tx, the formation of the Schottky junction could effectively accelerate the separation and migration of photogenerated electrons and holes, and thus boost the photocatalytic H2 evolution activity. In particular, among Ti3C2Tx (Tx = F, OH, O), the work function of Ti3C2Ox was the largest, and the Ti3C2Ox showed the strongest electron capture ability from ZnIn2S4. Experimental characterization analyses also demonstrated the rapid separation and transfer of photogenerated electrons and holes of ZnIn2S4/Ti3C2Ox. This work paves the way for photocatalytic applications of MXene-based photocatalysts by tuning their surface termination groups.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma16062168/s1, Figure S1: XRD patterns of the Ti3AlC2 and the as-synthesized Ti3C2Tx (Tx = F, OH, O); Figure S2: SEM of (a) Ti3C2Fx, (b) Ti3C2(OH)x and (c)Ti3C2Ox; Figure S3: XRD pattern of ZnIn2S4 and ZnIn2S4/Ti3C2Tx (Tx= F, OH, O); Figure S4: The SEM of ZnIn2S4; Figure S5: Electrostatic potentials of Ti3C2Tx (Tx = F, OH, O); Figure S6: Mott–Schottky diagram of ZnIn2S4; Table S1: the atomic ratio of Ti3C2Tx (Tx = F, OH, O) by XPS results [37,38].

Author Contributions

M.C.: Conceptualization, writing-review and editing and visualization; X.Z.: investigation and writing-original draft preparation; Z.Z.: methodology and Resources; J.B.: validation; Q.W.: data curation; Q.C.: data curation; funding acquisition; Y.W. and S.S.: project administration and funding acquisition. X.Z. contributed equally with M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by National Natural Science Foundation of China (Grant No. U1832165, 21902001 and 22102001), Provincial Natural Science Foundation of Anhui (Grant No. 2008085QB85 and 2108085QB48), Key Research and Development Program of Anhui Province (Grant No. 202004a05020015 and 006233172019), and Higher Education Natural Science Foundation of Anhui Province (KJ2021A0027 and KJ2021A0029).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Materials 16 02168 sch001 550
Scheme 1. Illustration for the formation of ZnIn2S4/Ti3C2Tx (Tx = F, OH, O) photocatalysts.
Scheme 1. Illustration for the formation of ZnIn2S4/Ti3C2Tx (Tx = F, OH, O) photocatalysts.
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Figure 1. (a) F 1s XPS spectra, (b) Ti 2p XPS spectra, (c) O 1s XPS spectra and (d) FT-IR spectra of Ti3C2Tx (Tx = F, OH, O).
Figure 1. (a) F 1s XPS spectra, (b) Ti 2p XPS spectra, (c) O 1s XPS spectra and (d) FT-IR spectra of Ti3C2Tx (Tx = F, OH, O).
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Figure 2. (a) SEM image, (b) TEM image, (c) HRTEM and (d) STEM image and corresponding EDX element mapping of Zn, In, S, Ti and C of ZnIn2S4/Ti3C2Ox.
Figure 2. (a) SEM image, (b) TEM image, (c) HRTEM and (d) STEM image and corresponding EDX element mapping of Zn, In, S, Ti and C of ZnIn2S4/Ti3C2Ox.
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Figure 3. (a) UV-vis DRS spectra of ZnIn2S4 and ZnIn2S4/Ti3C2Tx, and (b) Tauc’s bandgap plot.
Figure 3. (a) UV-vis DRS spectra of ZnIn2S4 and ZnIn2S4/Ti3C2Tx, and (b) Tauc’s bandgap plot.
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Figure 4. (a) Time course of photocatalytic H2 evolution performance of ZnIn2S4, and ZnIn2S4/Ti3C2Tx samples, (b) comparison of the photocatalytic H2 evolution rate of ZnIn2S4/Ti3C2Ox samples with different Ti3C2Ox contents and (c) recycling test of photocatalytic H2 production over ZnIn2S4/Ti3C2Ox.
Figure 4. (a) Time course of photocatalytic H2 evolution performance of ZnIn2S4, and ZnIn2S4/Ti3C2Tx samples, (b) comparison of the photocatalytic H2 evolution rate of ZnIn2S4/Ti3C2Ox samples with different Ti3C2Ox contents and (c) recycling test of photocatalytic H2 production over ZnIn2S4/Ti3C2Ox.
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Figure 5. (a) PL spectra, (b) time-resolved PL spectra, (c) EIS Nyquist plots and (d) transient photocurrent responses of ZnIn2S4 and ZnIn2S4/Ti3C2Tx (Tx = F, OH, O) samples.
Figure 5. (a) PL spectra, (b) time-resolved PL spectra, (c) EIS Nyquist plots and (d) transient photocurrent responses of ZnIn2S4 and ZnIn2S4/Ti3C2Tx (Tx = F, OH, O) samples.
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Figure 6. UPS spectrum of (a) ZnIn2S4, (b) Ti3C2Fx, (c) Ti3C2(OH)x and (d) Ti3C2Ox.
Figure 6. UPS spectrum of (a) ZnIn2S4, (b) Ti3C2Fx, (c) Ti3C2(OH)x and (d) Ti3C2Ox.
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Figure 7. Schematic illustration for the photocatalytic H2 evolution reaction over ZnIn2S4/Ti3C2Ox.
Figure 7. Schematic illustration for the photocatalytic H2 evolution reaction over ZnIn2S4/Ti3C2Ox.
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Cai, M.; Zha, X.; Zhuo, Z.; Bai, J.; Wang, Q.; Cheng, Q.; Wei, Y.; Sun, S. Enhanced Photocatalytic Hydrogen Production of ZnIn2S4 by Using Surface-Engineered Ti3C2Tx MXene as a Cocatalyst. Materials 2023, 16, 2168. https://doi.org/10.3390/ma16062168

AMA Style

Cai M, Zha X, Zhuo Z, Bai J, Wang Q, Cheng Q, Wei Y, Sun S. Enhanced Photocatalytic Hydrogen Production of ZnIn2S4 by Using Surface-Engineered Ti3C2Tx MXene as a Cocatalyst. Materials. 2023; 16(6):2168. https://doi.org/10.3390/ma16062168

Chicago/Turabian Style

Cai, Mengdie, Xiaoqing Zha, Zhenzhen Zhuo, Jiaqi Bai, Qin Wang, Qin Cheng, Yuxue Wei, and Song Sun. 2023. "Enhanced Photocatalytic Hydrogen Production of ZnIn2S4 by Using Surface-Engineered Ti3C2Tx MXene as a Cocatalyst" Materials 16, no. 6: 2168. https://doi.org/10.3390/ma16062168

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